1. Field of the Invention
The present invention relates to an optical element and a light emitting device.
2. Description of Related Art
In these years, LED lighting devices are coming into practical use in accordance with the development of high power LEDs (Light Emitting Diodes) and high efficiency LEDs as alternatives to incandescent light-bulbs and fluorescent lamps. Being compared with an incandescent light-bulb and a fluorescent lamp, an LED is small in its size and high in the luminous density. Furthermore, while the incandescent light-bulb and fluorescent lamp emit light in all the directions, the LED is provided with a feature of having a narrow directivity of its light. Moreover, recently power LEDs with their capacity of 3 W and even those with the capacity of 10 W have been also coming into practical use.
As a plane lighting unit using such an LED for a light source, proposed is a plane lighting device described below. Specifically, a light guiding plate includes a light incoming section for guiding light from an LED, a surface section for emitting the light, a rear surface section located at a side opposite to the surface section, and a side section connecting the surface section and the rear surface section; in which the light incoming section is located at a position inner than the side section, and optical elements (prisms) provided with tilted surfaces corresponding to a bundle of light coming from the LED are placed in the surface section continuously at positions corresponding to a brightness distribution or an optical energy distribution of a light source. The prisms are provided with a density distribution increasing functionally in proportion to a distance from the LED (Refer to Patent Document 1).
In the plane lighting device described in Patent Document 1, intervals of the prisms, or a distribution of the prisms in other words, are dense, for example at a light guiding section that is located distant from the LED and likely to have a low brightness. Through such an arrangement, a uniform brightness and a uniform energy emission are achieved at the emission side, and light can be emitted in such a way that a product of an amount of emitted light and an emission light angle is uniform at any position in the light guiding plate.